The instant invention teaches the use of a sense oscillator in conjunction with a loop antenna. When a metal object or the like enters the normal loop field, the effective inductive reactance will be changed. The result is the oscillator frequency changes in proportion to the object's effect on the loop's electromagnetic field lines.
The use of such a sense oscillator arrangement, resulting in a change in oscillator frequency is well known as a beat frequency oscillator (BFO) metal detector and is particularly taught in U.S. Pat. No. 2,442,805 by Gilson entitled "Metal Locator", and more recently in U.S. Pat. No. 4,196,391 by Weber entitled "Metal Locator with Stereotonic Indication of Translateral Position". In each of these prior inventions, the sense oscillator output signal is heterodyned with a separate beat oscillator. The result is a difference frequency which varies at an audio rate equal to the change in hertz of the sense oscillator frequency produced by the obscured object. The effective rate of change may be increased: doubled, or tripled at least, through the expedient of harmonic mixing, e.g. having a second or higher order harmonic of the sense oscillator heterodyning with a higher frequency beat oscillator. Such an enhancement scheme is taught by Speaker in U.S. Pat. No. 2,393,717 entitled "Electronic Surgical Metal Body Locator", as well as more recently by Weber in U.S. Pat. No. 4,255,710 entitled "Directional Metal Detector Apparatus Having Enhanced Sensitivity". Each of these prior teachings provide a good increase in sensitivity over what fundamental frequency mixing produces.
The sensitivity limitations of this prior art is overcome by the instant invention, while still retaining most of the intrinsic advantages of the BFO class of metal detectors. The sense oscillator and inducer loop antenna coact to produce a frequency change in the sense oscillator output in a manner virtually the same as with any ordinary BFO metal detector. Therefore, the interactive effect between the sense oscillator is well established through long experience. The important advance wrought by this invention occurs after the sense oscillator frequency pertubations are produced. No beat frequency oscillator is used. This eliminates the instability, e.g. drift, etc., inherent in the usual beat oscillator. Frequency "pulling" between the sense oscillator and a beat oscillator, especially when near zero beat, is eliminated. Most importantly, the intrinsic limitation that the audio beat frequency note change rate is proportional to the relationship between the sense and beat oscillator is no longer a limiting sensitivity factor.
The instant invention uses a frequency shift detector to develop an automatic frequency control signal which reacts to any change in the sense oscillator frequency. The result is an error signal which can be amplified to any reasonable extent, then used to control the rate of a voltage controlled oscillator operating at an audio rate. The result, due to the possible amplification factors involved, is a rate of apparent output pitch change which is much more exagerated than what the change in sense oscillator rate may alone produce.
Therefore it is a purpose of the invention to provide a metal detector having a sense oscillator coupled with an inducer loop.
Another purpose of the invention is to provide a detector responsive to any frequency shift produced in the sense oscillator by presence of metal near the loop antenna, with the detector producing an error signal therefrom which is proportional to the shift.
Yet another intent is to provide a metal detector having enhanced sensitivity.
The invention's essence rests in the coaction of a sense oscillator frequency pertubations with a frequency shift detector so as to produce an error signal which serves to control rate of an audio frequency voltage controlled oscillator resulting in an aural output indication therefrom.
Still another point is to provide a directional, stereoresponsive metal detector providing "right" and "left" sense.
Given these teachings, the invention provides a sensitive metal detector suited for locating hidden objects, treasures, pipes, cables, weapons, shrapnel, and a variety of other such items.
Furthermore, the metal detector can be operated to serve to discriminate between materials having different magnetic properties.